Rheometers are used to study flow and deformation of materials. Rotary rheometers can typically hold a sample between a fixed base plate and a rotating upper plate. In one type of measurement, the gap between the base and rotating upper part is kept constant, and in another, the normal force on the sample is kept constant.
Normal force control has typically been implemented by a simple on-off modulation approach. Upper and lower pass bands are set around the desired normal force loop. A simple step size and update rate are set and the system moves and measures until the force is in the pass band. The control then switches off until the force is seen to move out of the pass bands.